The current arising from charged point particles must be mapped onto the Maxwell
propagation grid. This requires a smearing or regularization function $\phi(\mathbf{r})$ attached to
each particle position $\mathbf{r}_i$ with user defined cutoff width, $\sigma$
Options:
COS2:
$\phi(r)=\text{cos}^2(\frac{\pi}{2}\frac{|\mathbf{r}-\mathbf{r}_i|}{\sigma})$
if $|\mahtbf{r}-\mathbf{r}_i|<\sigma$, and 0 otherwise.